The present invention relates to methods and apparatus for depositing useful layers of materials on substrates used in the manufacture of semiconductor die. More particularly, the invention relates to improved apparatus and methods for use in the processing of substrates, such as by chemical vapor deposition.
Chemical vapor deposition, commonly referred to as "CVD," is used to deposit a thin layer of material on a semiconductor substrate. To process substrates with the CVD process, a vacuum chamber is provided with a susceptor configured to receive a substrate thereon. In a typical prior art CVD chamber, the substrate is placed into and removed from the chamber by a robot blade. The chamber includes an intermediate substrate positioning assembly on which the substrate is located when it is placed into, or about to be removed from, the chamber. To locate the substrate on the susceptor, the susceptor is passed through the center of the substrate positioning assembly to lift the substrate therefrom. The susceptor and the substrate are then heated to a temperature of between 250.degree.-650.degree. C. Once the substrate is located on the susceptor and heated to an appropriate temperature, a precursor gas is charged to the vacuum chamber through a gas manifold situated above the substrate. The precursor gas reacts with the heated substrate surface to deposit the thin material layer thereon. As the gas reacts to form the material layer, volatile byproduct gasses are formed, and these gasses are pumped out of the vacuum chamber through a chamber exhaust system.
A primary goal of substrate processing is to obtain as many useful die as possible from each substrate. Many factors influence the processing of substrates in the CVD chamber and affect the ultimate yield of die from each substrate processed therein. These factors include processing variables, which affect the uniformity and thickness of the deposition material layer deposited on the substrate, and contaminants that can attach to the substrate and contaminate one or more die therein. Both of these factors must be controlled in CVD and other processes to maximize the die yield from each substrate.
One CVD processing variable which affects the uniformity of the deposition material layer is the relative concentration of reacted and non-reacted process gas components in the deposition chamber. The exhaust system of the chamber includes a circumferential exhaust channel located above the substrate adjacent the perimeter thereof, through which the reacted process gas is vented. However, a gap exists in the circumferential exhaust channel where the slit valve for moving the substrate into and out of the chamber passes through the chamber wall. The exhausting of reacted gaseous products from the chamber is less efficient near this gap, and thus the exhausting of reaction products is non-uniform in the chamber. This contributes to the creation of non-uniform deposition material layers on the substrate, because the relative concentration of reactive gas in the total gas volume in the chamber varies about the surface of the substrate due to the non-uniform exhausting of reacted gaseous products from the chamber.
In addition to the foregoing factor which affects the uniformity and thickness of the deposition material layer, CVD processing chambers include multiple sources of particle contaminants which, if received on the substrate, reduce the die yield therefrom. One primary source of particulate contamination in CVD processing is the deposition material deposited on the chamber surfaces during processing. As the substrate is processed in the CVD chamber, a material layer is indiscriminately deposited on all surfaces within the chamber which are contacted with the gas, such as the aforementioned lamp covers. If these chamber surfaces are later touched or rubbed, or if the material layer is loosely attached to the chamber surface and the chamber is shaken or vibrated, particles of deposition material layer can become free in the chamber and contaminate the substrate. Additionally, the deposition material layer does not typically firmly attach itself to the edge and underside of the substrate, and the layer formed in that location of the substrate is known to flake off the substrate and become a particle contaminant.
One method of controlling particle generation in the chamber is to use a shadow ring to reduce the occurrence of the deposition layer on the edge and underside of the substrate. A shadow ring is a masking member, which is received on the susceptor and contacts the upper, outer, circumferential area of the substrate and limits access of the deposition gas to the contacted area of the substrate. However, the shadow ring has several, limitations which contribute to the non-uniform processing of substrates. The volatile deposition gas still tends to migrate under the lip of the shadow ring and deposit a material layer on the substrate edge and underside which may later flake off. Additionally, the engagement of the shadow ring with the substrate can create particles. Finally, the shadow ring is a heat sink, which draws heat out of the substrate and thus reduces the temperature of the substrate adjacent the area of contact between the substrate and shadow ring, which affects the thickness of the deposition material layer on the area of the substrate adjacent the shadow ring.
One alternative to the shadow ring is disclosed in European Patent Application No. EPO 467 623 A3, published Jan. 22, 1992. In that application, a shroud is provided around the perimeter of the substrate. The shroud includes a lip which overhangs, but does not touch, the substrate. A gas is provided to the underside of the substrate, and a portion of this gas flows outwardly between the substrate and susceptor and into a gap formed between the substrate and the shroud. Although the shroud creates a circumferential channel in which a non-reactive gas may be maintained to mask the edge of the substrate, the structure shown in EPO 467 623 A3 has several disadvantages. First, when the shroud is received on the susceptor, it aligns with the susceptor and no means is disclosed for aligning the substrate with the shroud. Any substantial misalignment between the substrate and the susceptor will result in substantial misalignment between the shroud and substrate, and the resulting annular gap between the substrate and shroud will be non-uniform around the perimeter of the substrate. This will create differential masking gas flow in different locations of the substrate edge. Second, introduction of the masking gas inwardly of the outer diameter of the substrate can cause the substrate to float off the susceptor during processing if the chamber pressure and process gas flow are not closely controlled. Finally, the European application notes that the shroud disclosed therein masks the upper surface of the substrate and prevents depositions thereon, which reduces the useful area of the substrate.
A further source of substrate particulate contamination occurs where a cracked, warped or substantially misaligned substrate is present in the chamber. If a cracked, warped or substantially misaligned substrate is encountered, substantial numbers of particulate contaminants can be generated as the substrate is moved in the chamber. Additionally, if large segments of the substrate become free in the chamber, they may seriously damage the chamber components. Finally, the upper surface and passageways of the susceptor could be exposed to the corrosive reactive gas if a cracked, warped or misaligned substrate is processed in the chamber.